Cyclen-conjugated rhodamine hydroxamate as Pd2+-specific fluorescent chemosensor

Article abstract of DOI:10.1002/asia.201100126

Youa're my hydroxamate: A chemosensor based on rhodamine hydroxamate with cyclen-tri (tert-butyl ester) and pyridine moiety binding units binds selectively with Pd²⁺ to induce a strong fluorescence enhancement and color change in aqueous soluti

Full text of DOI:10.1002/asia.201100126

DOI: 10.1002/asia.201100126
2
+
Cyclen-Conjugated Rhodamine Hydroxamate as Pd -Specific Fluorescent
Chemosensor
[
a]
Hyemi Kim, Kyung-Soo Moon, Soyoung Shim, and Jinsung Tae*
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
Palladium is widely used in various materials, such as
dental crowns, fuel cells, jewelry, automobile, and catalysts.
chemosensors show interference by platinum species owing
to their similar chemical properties.
[1]
And palladium-catalysed reactions, such as Buchwald–Hart-
wig, Heck, Sonogashira, and Suzuki–Miyaura reactions, are
increasingly important because of their advantages in form-
Over the past few years, we have developed fluorescent
sensing systems for the selective detection of metal ions
[8]
using rhodamine–amide fluorophores. Whilst the ring-
closed spirolactam form is nonfluorescent and colorless, the
corresponding open form is strongly fluorescent and pink
colored. Judicious choice of ligand attached to the spirolac-
tam ring could selectively bind with a metal ion to induce
ring opening and eventually turn on the fluorescence signal.
Cyclen derivatives are commonly used as ligands owing to
their strong coordination ability to many transition metal
[2,7a]
ing difficult bonds.
tion steps, residual palladium species, which can be hazard-
However, even after several purifica-
[3]
ous to human health, are often found in the final products.
In particular, owing to its ability to coordinate with DNA,
thiol-contaning amino acids, proteins, and vitamin B , palla-
6
dium is known to potentially disturb several cellular process-
[
4]
es. Because the proposed dietary intake is between<1.5
À1
[9]
and 15 mgday per person and the threshold for palladium
ions. Recently, a few research groups have reported
in drugs is <5–10 ppm, the products obtained after palladi-
cyclen-linked fluorescent chemosensors for the detection of
[10]
um-catalyzed reactions need extensive purification and anal-
metal ions. The pyridine moiety is also well known as a
[4,5]
[11]
ysis.
good ligand for metal ions. Thus, we envisioned that the
Therefore, analytical methods are urgently needed for the
sensitive and selective detection of palladium in a high-
throughput fashion. The typically used analytical methods,
such as atomic absorption spectrometry, plasma emission
spectroscopy, solid-phase microextraction high-performance
liquid chromatography, X-ray fluorescence, etc., require
introduction of a cyclen moiety and a pyridine into the
rhodamine hydroxamate could provide a well-organized co-
ordination platform for metal ions, as shown in Scheme 1.
The rhodamine–cyclen probe (1) was prepared in two steps:
1) 2,6-bis(bromomethyl)pyridine, K CO , CH CN, RT, 2) 2,
2
3
3
Na CO , CH CN, reflux, from the known rhodamine hy-
2
3
3
[6]
[8c]
high cost and heavy instrumentation. A fluorescent che-
mosensor method would be more desirable because it is less
labor-intensive and highly sensitive. Recently, a few fluores-
cent-sensing systems for palladium species utilizing either
coordination-based sensing methods or palladium-catalyzed
droxamic acid 3 (Scheme 1).
[7]
chemical reactions have been reported. Most palladium
[
a] H. Kim, K.-S. Moon, S. Shim, Prof. J. Tae
Department of chemistry and
Center for Bioactive Molecular Hydrids (CBMH)
Yonsei University
Seoul 120-749 (Korea)
Fax : (+82)2-364-7050
E-mail: jstae@yonsei.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100126.
Scheme 1. Synthesis of the rhodamine-cyclen conjugate 1.
Chem. Asian J. 2011, 6, 1987 – 1991
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1987

COMMUNICATION
2
+
Probe 1 shows neither color nor fluorescence in H O
DMSO 1%, v/v) solution indicating that it exists in the spi-
onstrates that the detection of Pd ions is possible at the
85 nm level. Under these conditions, the fluorescence inten-
sity of the solution of 1 is linearly proportional to the con-
2
(
rocyclic form predominantly as expected. On treatment with
2
+
2+
5
.0 equiv of Pd ions, probe 1 (10 mm) exerts strong fluores-
centration of Pd ions (Figure 1b).
shows that 1 forms a 1:2 complex with
Pd ions (see the Supporting Information). The binding
[12]
cence signal at 581 nm in H O (DMSO 1%, v/v). In addi-
The Job plot
2
2
+
tion, the solution changes from colorless to pink-red color
2
+
2+
upon addition of Pd ions. Probe 1 monitors Pd ions in
the pH 7–9 range (see the Supporting Information).
constant (log K=10.26), which was calculated by the
[13]
Benesi–Hildebrand method
in water (DMSO 1%, v/v)
A fluorescence titration experiment of 1 (10 mm) with
from the fluorescence titration experiments based on a 1:2
binding model, showed strong binding bewteen 1 and Pd
2
+
2+
Pd
58C. Upon each addition of Pd ions, the solution was in-
cubated for 30 minutes and then fluorescence intensity was
ions was conducted in water (DMSO 1%, v/v) at
2
+
2
ions. The addition of excess sodium cyanide to a mixture of
2
+
1 and Pd ions decreases the fluorescence intensities of the
solution and leads to disappearance of the pink color (see
the Supporting Information). This result implies the reversi-
2
+
measured. About 2.0 equivalents of Pd ions were required
for saturation of the fluorescence intensity under the titra-
tion conditions (Figure 1a). The UV/Vis absorption spec-
trum of 1 shows a distinctive absorption at 480–610 nm with
a clear color change from colorless to pink in the presence
2
+
ble binding properties of 1 with Pd ions in aqueous solu-
tions.
To evaluate the selectivity of probe 1, fluorescence re-
sponses of 1 (10 mm) to other biologically relevant metal
ions in water (DMSO 1%, v/v) were examined. Upon addi-
2
+
of Pd ions (see the Supporting Information). The fluores-
2
+
cence titration experiment of 1 at 10 mm with Pd ions dem-
3
+
2+
3+
2+
tion of 5.0 equivalents of metal ions (Fe , Fe , Au , Hg
2
+
2+
2+
2+
2+
2+
2+
2+
3+
,
Zn , Pb , Ca , Co , Mn , Mg , Cu , Cd , Al ,
3
+
+
+
+
+
2+
3+
2+
2+
+
Cr , Au , Ag , Na , Li , Pt , Ru , Rh , Ni , K , and
2
+
2+
Ba ) at 258C, only Pd ions induced significant fluores-
cence-intensity enhancement. Other metal ions did not
cause any significant changes in fluorescence intensity (Fig-
ure 2a). This result implies that the selectivity of 1 toward
Pd ions over other metal ions is extremely high. More im-
portantly, competitive Pt ions afforded no enhancement in
fluorescence intensity. Fluorescence interference by the
presence of other metal ions was negligible, except for Hg
ions which are known to bind strongly with the cyclen
[14]
2
+
2
+
2
+
[10c]
moiety (Figure 2b).
The fluorescence selectivity and colorimetric selectivity of
1
were well-matched. Whilst the binding of 1 (10 mm) with
2
+
Pd ions resulted in obvious colorless to pink-red color
changes in aqueous solutions, no significant color changes
were promoted by other metal ions (Figure 3).
To confirm the cooperative effects of the pyridine and the
2
+
cyclen moieties of 1 for the selective binding with Pd ions,
we tested the corresponding rhodamine hydroxamates with
either a pyridine or a cyclen moiety (see the Supporting In-
formation). These model compounds exerted poor metal-ion
2
+
selectivities and very weak binding properties with Pd
ions, which implies that the two binding sites, the pyridyl
and the cyclen-tri(tert-butyl ester) groups, of 1 bind coopera-
2
+
tively with Pd
ions to display the observed selectivity.
Scheme 2 shows a proposed binding complex, where all
three binding groups participate in the complexation with
2
+
Pd ions.
Next, we conducted experiments to detect palladium com-
plexes in aqueous solutions to demonstrate the potential ap-
plications of this palladium-ion probe. We first compared
fluorescence intensity changes of 1 induced by different pal-
2
+
Figure 1. Fluorescence intensity changes of 1 upon addition of Pd in
O (DMSO 1%, v/v). Each spectrum is measured 30 min after each
H
2
2
+
Pd addition at 258C with excitation at 520 nm. a) 1 (10 mm). Inset: plot
of fluorescence intensities at 581 nm depending on the equivalents of
Pd . b) 1 (10 mm) upon addition of Pd (by 85 ppb). Inset: plot of fluo-
rescence intensities at 578 nm depending on the concentration of Pd
ions.
ladium complexes (PdCl , Pd ACHUTNTGRENNNU(G NO ) , [K PdCl ], Pd ACHTUNGTRENNUNG( OAc) ,
3 2 2 4 2
2
2
+
2+
and [Pd ACHTUNGTRNENUG( PPh ) ]). The palladium complexes were dissolved
3 4
in dimethyl sulfoxide (10 mm) at 258C and then added to
the solutions of 1 (10 mm) in water (DMSO 1%, v/v). The
2
+
1988
www.chemasianj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1987 – 1991

2
+
Pd -Specific Fluorescent Chemosensor
2
+
Scheme 2. Proposed structure of the 1:2 complex between 1 and Pd
ions.
resultant solutions were incubated for 30 minutes at 258C
prior to fluorescence measurements. It was possible to
detect most palladium(II) complexes in aqueous solutions
by fluorescence and colorimetry methods (Figure 4). Al-
though palladium(0) complex [Pd ACTHUNGTRNEUNG( PPh ) ] shows relatively
3 4
small fluorescence intensity changes, it is still possible to
detect palladium(0) complexes in aqueous solutions.
2
Figure 2. Fluorescence intensity changes of 1 (10 mm) in H O (DMSO
1
2
1
1
%, v/v) at 581 nm. a) In the presence of metal ions (5.0 equiv): 1 none,
2
+
3+
2+
2+
2+
2+
2+
2+
2+
Pd , 3 Fe , 4 Fe , 5 Hg , 6 Zn , 7 Pb , 8 Ca , 9 Co , 10 Mn
,
2
+
2+
2+
3+
2+
+
+
+
1 Mg , 12 Cu , 13 Cd , 14 Al , 15 Cr , 16 Ag , 17 Na , 18 Li ,
2
+
3+
2+
2+
+
2+
9 Pt , 20 Ru , 21 Rh , 22 Ni , 23 K , 24 Ba . b) In the presence of
2
+
3+
2+
2+
Figure 4. a) Fluorescence intensity changes and b) color changes of 1
Pd ions (5.0 equiv) and other metal ions (5.0 equiv). 1 none, 2 Fe
,
,
,
2
+
2+
2+
2+
2+
2+
2+
(10 mm) in the presence of palladium complexes (5.0 equiv) in H
(DMSO 1%, v/v) at 258C (at 578 nm): 1 none, 2 PdCl , 3 Pd(NO
4 [K PdCl ], 5 Pd(OAc) , 6 [Pd(PPh ].
2
O
3
1
1
Fe , 4 Hg , 5 Zn , 6 Pb , 7 Ca , 8 Co , 9 Mn , 10 Mg
+ + + +
2
2+
3+
3+
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3 2
) ,
1 Cu , 12 Cd , 13 Al , 14 Cr , 15 Ag , 16 Na , 17 Li , 18 Pt
3
+
2+
2+
+
2+
2
4
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
U
3 4
)
9 Ru , 20 Rh , 21 Ni , 22 K , 23 Ba
.
2
+
Next, we carried out experiments to detect residual Pd
ions in glass reactors.
[7f]
Solutions of each palladium(II)
(OAc) in tet-
complex, PdCl , Pd(NO ) , [K PdCl ], and PdACTHUNGTRENNUNG
A
H
U
G
R
N
U
G
2
3
2
2
4
2
rahydrofuran were stirred in a flask for 3 hours at room
temperature, then the flask was cleaned according to the
usual laboratory washing procedure (brushed with deter-
gent, washed with water and acetone, then oven-dried). A
solution of probe 1 in water (DMSO 1%, v/v) was added to
the cleaned flask and stirred overnight at room temperature
prior to fluorescence measurements (see the Supporting In-
formation). The observed fluorescence intensities of the pre-
pared samples showed that most of the flasks still had signif-
icant amounts of residual palladium species (Figure 5). This
result implies that this probe system could be effectively
Figure 3. Color changes of 1 (10 mm) in the presence of metal ions
3
+
2+
2+
2+
3+
(
5.0 equiv) in H
2
O (DMSO 1%, v/v): 1 none, 2 Fe , 3 Fe , 4 Hg
,
,
,
2
+
2
+
2+
2+
2+
2+
2+
2+
used to detect residual Pd complexes in aqueous solutions.
5
1
2
Zn
,
6 Pb
,
7 Ca
,
8 Co
,
9 Mn
,
10 Mg ,11 Cu , 12 Cd
2
+
3
+
3+
+
+
+
2+
2+
In conclusion, we have developed a Pd -selective chemo-
sensor based on rhodamine hydroxamate with a cyclen-tri-
3 Al , 14 Cr , 15 Ag , 16 Na , 17 Li , 18 Pd , 19 Pt , 20 Ru
2
+
2+
+
2+
1 Rh , 22 Ni , 23 K , 24 Ba
.
Chem. Asian J. 2011, 6, 1987 – 1991
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1989

J. Tae et al.
COMMUNICATION
Acknowledgements
This work was supported by CBMH (Yonsei University) through the Na-
tional Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (MEST) No. 2011-0001126.
Keywords: chemosensor · fluorescent probes · palladium ·
rhodamine · cyclen
2
Figure 5. a) Fluorescence intensities of 1 (20 mm) in H O (DMSO 1%, v/
v) prepared from the flasks contaminated by the following palladium
complexes: 1 none, 2 PdCl 3 Pd(NO 4 [K PdCl ], 5 Pd(OAc)
b) Fluorescence intensities at 578 nm.
2
,
A
H
U
G
R
N
U
3
)
2
,
2
4
A
H
U
G
E
N
N
2
.
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tert-butyl ester) and a pyridine moiety as binding units. This
2
+
chemosensor binds specifically with Pd ions to induce a
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+
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General Methods
Acetonitrile was distilled over calcium hydride under nitrogen immedi-
ately prior to use. All reactions were carried out under a nitrogen atmos-
phere. Chromatographic purification were performed on silica gel (230–
2
007, 79, 6383–6389.
0
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00 mesh) with the solvent systems indicated. All recorded melting
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II
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e. The cyclen derivative 2 was prepared according to the known proce-
[
8c]
IV
0
II
4
6, 1079–1081. For a Pd ion sensor (also detects Pd and Pd
[
15]
dure.
,6-Bis(bromomethyl)pyridine (70 mg, 0.24 mmol) and potassium carbon-
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3
:1 to 1:1, v/v) to give 36 mg (0.06 mmol, 47%) of white solid.
Sodium carbonate (25 mg, 0.24 mmol) and compound
2
(60 mg,
0
0
4
.12 mmol) were added to a solution of the white solid (75 mg,
.12 mmol) in acetonitrile (5 mL). The solution was heated at reflux for
8 h and the reaction mixture was filtered and concentrated in vacuo.
The crude product was purified by column chromatography on silica gel
CH Cl /MeOH: 10/1 to 9/1, v/v) to give 1 (119 mg, 0.11 mmol, 92%) as
white solid: =0.33 (CH Cl /MeOH=10:1); m.p. 130–1358C;
H NMR (400 MHz, CDCl ): d=7.85 (d, J=6.8 Hz, 1H), 7.47–7.43 (m,
H), 7.07–6.98 (m, 3H), 6.37 (d, J=8.8 Hz, 2H), 6.31(s, 2H), 6.17 (d, J=
(
2
2
a
R
f
2
2
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1
3
3
8
2
1
1
1
1
1
1
1
ACHTUNGTRENNUNG
.4 Hz, 2H), 4.84 (s, 2H), 3.57 (bs, 2H), 3.32 (q, J=7.2 Hz, 8H), 3.17–
.29 (broad, 22H), 1.43 (s, 9H), 1.31 (s, 18H), 1.17 ppm (t, J=7.2 Hz,
1
3
2H); C NMR (100.6 MHz, CDCl
3
): d=172.6, 165.8, 158.0, 155.9, 153.5,
51.0, 148.7, 136.9, 133.3, 128.7, 128.4, 124.1, 122.7, 122.5, 122.2, 107.9,
05.0, 97.7, 82.2, 82.1, 78.8, 65.6, 59.5, 56.4, 55.7, 50.8, 44.3, 28.0,
2.8 ppm; IR (film): n˜ =2972, 2932, 2900, 2837, 2450, 2359, 2337, 1721,
634, 1612, 1544, 1512, 1454, 1427, 1368, 1310, 1265, 1224, 1157, 1116,
À1
+
076, 1017, 751 cm ; HRMS
A
H
U
G
R
N
U
G
H
86
N
8
O
9
(M+Na )
097.6415; found 1097.6409.
1990
www.chemasianj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1987 – 1991

2
+
Pd -Specific Fluorescent Chemosensor
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Finn, Chem. Commun. 2007, 1269–1271.
[
2
Received: February 9, 2011
Published online: April 19, 2011
3
+
[
14] Gd does not induce any fluorescence intensity changes of 1 in an
aqueous solution (see the Supporting Information).
Chem. Asian J. 2011, 6, 1987 – 1991
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1991